FW: FW: Question about the Ignore Wide Residue message
sk_jones at qlc.com
Mon Jun 6 06:58:00 PDT 1994
This is our take.
From: Gary on Mon, Jun 6, 1994 2:50 PM
Subject: Re: FW: Question about the Ignore Wide Residue message
To: sk_jones at qlc.com
This is inreply to the message that you forwarded me from
shiang at corp.cirrus.com
in regards to the Ignore Wide Residue message. I have not forwarded this to
The questions were:
1. Why does this message only apply to the target side of a SCSI bus.
The target sends this message to the initiator, but the initiator does
not send this message to the target.
Also, is it necessary for a target to send this message when the DATA
IN is a read command, since the initiator already knows that the block
length is not a multiple of the scsi width?
2. In what commands does the target send this message to the
The answers are:
1. The message applies only to the target side of the SCSI bus, because
the target is the side of the bus that determines the amount of data
actually moved on the SCSI bus.
It is necessary for the target to send this message whenever the number
of "valid" data bytes does not match the current (negotiated) bus
If the block length is 511 and 3 blocks are sent accross a 16 bit
data path, then the target must send the Ignore Wide Residue message
because 8 bits of the last trasfer is invalid. The block size is
determined by the target. I don`t know of any target devices that
support Wide SCSI that also support odd block sizes (511, 1025, ...),
but SCSI defines a protocol to handle this.
2. The type of commands that the target may need to send this message will
typically be Inquiry, Request Sense, and Mode Sense. The amount of
data returned by these commands is typically not known by the initiator
and is not required (by SCSI) to be divisible by the current negotiated
bus width. The CDB setup by the initiator only specifies a maximum
exceptable transfer length (allocation length). Many initiators will
set the allocation length to the maximum value of 255, the actual data
length is deteremined by the target.
As an example: a 33 byte mode sense page is 100% legal SCSI and if
the current negotiated bus width is other than 8 bits, an Ignore Wide
Residue message is in order.
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